The present invention relates to an electrophotographic image forming apparatus, such as a copying machine, a printer, a facsimile, or a multifunction apparatus formed by combining them, and in particular, to an improvement in a cleaning technique for an image forming apparatus.
Up to now, electrophotographic (electrostatic transfer type) image forming apparatuses, such as copying machines and printers, have been widely known. In such image forming apparatuses, a toner image is transferred to a recording sheet by a final transfer member, and then the toner image is fixed to thereby realize on the recording sheet the toner image as a permanent image. The portion of the toner remaining on the transfer member without being transferred to the recording sheet has to be removed by a cleaning device in the image forming apparatus. As a method for removing such residual toner, there has been proposed a technique according to which a metal cleaning roll is caused to abut the transfer member, thereby making the residual toner to adhere to the cleaning roll by electrostatic force (See, for example, JP 6-59586 A and JP 6-35340 A). Further, there has also been proposed a technique according to which a metal cleaning blade is caused to abut against a metal cleaning roll to prevent the cleaning blade from being turned up (See, for example, JP 2000-142310).
However, these conventional techniques have the following problem.
The metal cleaning roll exhibits a relatively low electrical resistance value, so that the surface potential thereof is quickly changed in response to a variation in the bias to be applied thereto. Whereas, an intermediate transfer member which is in contact with the cleaning roll exhibits a relatively high electrical resistance value, so that even if the bias to be applied varies, the surface potential thereof is only changed transitionally. Thus, an excessive difference in potential can be instantaneously generated between the cleaning roll and the intermediate transfer member. As a result, a discharge stress is imparted to the intermediate transfer member, thereby deteriorating or damaging the surface of the intermediate transfer member.
The present invention has been made in view of the above-mentioned technical problem, and provides an image forming apparatus capable of preventing generation of an excessive difference in potential between a cleaning member and an intermediate transfer member, thereby making it possible to elongate the service life of the intermediate transfer member.
According to an aspect of the present invention, an image forming apparatus includes: an image bearing member; an image forming unit which forms a toner image on the surface of the image bearing member; an intermediate transfer device formed by at least one intermediate transfer member, and which is in contact with the image bearing member; a final transfer member which is in contact with the intermediate transfer member; a cleaning member which is in contact with the intermediate transfer member; and a control unit which applies an intermediate-transfer-member-bias to the intermediate transfer member and applies a cleaning-member-bias to the cleaning member which is in contact with the intermediate transfer member to thereby control a potential gradient between the cleaning member and the intermediate transfer member, in which when switching the intermediate-transfer-member-bias to be applied to the intermediate transfer member from a first intermediate-transfer-member-bias to a second intermediate-transfer-member-bias and switching the cleaning-member-bias to be applied to the cleaning member from a first cleaning-member-bias to a second cleaning-member-bias, respectively (substantially at the same time), the control unit makes the switching start timing for the intermediate-transfer-member-bias earlier than the switching start timing for the cleaning-member-bias.
More specifically, the control unit can be constructed such that during the transition of the intermediate-transfer-member-bias from the first intermediate-transfer-member-bias to the second intermediate-transfer-member-bias, the switching of the cleaning-member-bias is started. Further, the control unit can also be constructed such that after the transition of the intermediate-transfer-member-bias to the second intermediate-transfer-member-bias, the switching of the cleaning-member-bias is started.
According to another aspect of the present invention, the image forming apparatus includes: an image bearing member; an image forming unit which forms a toner image on the surface of the image bearing member; an intermediate transfer device formed by at least one intermediate transfer member, and which is in contact with the image bearing member; a final transfer member which is in contact with the intermediate transfer member; a cleaning member which is in contact with the intermediate transfer member; and a control unit which applies an intermediate-transfer-member-bias to the intermediate transfer member and applies a cleaning-member-bias to the cleaning member which is in contact with the intermediate transfer member to thereby control the potential gradient between the cleaning member and the intermediate transfer member, in which when switching the intermediate-transfer-member-bias to be applied to the intermediate transfer member from a first intermediate-transfer-member-bias to a second intermediate-transfer-member-bias and switching the cleaning-member-bias to be applied to the cleaning member from a first cleaning-member-bias to a second cleaning-member-bias, respectively (substantially at the same time), the control unit effects the switching of the cleaning-member-bias stepwise.
More specifically, the stepwise switching of the cleaning-member-bias may be effected in two stages; it is possible to effect switching from a first cleaning-member-bias to a transition cleaning-member-bias and then effect switching from an intermediate cleaning-member-bias to a second cleaning-member-bias. The stepwise switching of the cleaning-member-bias may be effected in three stages; it is possible to effect switching from a first cleaning-member-bias to a first transition cleaning-member-bias and then effect switching from the first transition cleaning-member-bias to a second transition cleaning-member-bias and then effect switching from the second transition cleaning-member-bias to a second cleaning-member-bias.
Further, it is possible to set the transition cleaning-member-biases (the first transition cleaning-member-bias and the second transition cleaning-member-bias) to be smaller than the second intermediate-transfer-member-bias.
Further, the stepwise switching of the cleaning-member-bias is effected such that the variation in the cleaning-member-bias increases gradually. For example, it is possible to effect setting such that the following inequality holds good: |(transition cleaning-member-bias)xe2x88x92(first cleaning-member-bias)| less than |(second cleaning-member-bias)xe2x88x92(transition cleaning-member-bias)|, and that the following inequality holds good: |(first transition cleaning-member-bias)xe2x88x92(first cleaning-member-bias)| less than |(second transition cleaning-member-bias)xe2x88x92(first transition cleaning-member-bias)| less than |(second cleaning-member-bias)xe2x88x92(second transition cleaning-member-bias)|.
Further, it is also possible to combine the first and second aspects of the invention. That is, according to another aspect of the present invention, the image forming apparatus includes: an image bearing member; an image forming unit which forms a toner image on the surface of the image bearing member; an intermediate transfer device formed by at least one intermediate transfer member, and which is in contact with the image bearing member; a final transfer member which is in contact with the intermediate transfer member; a cleaning member which is in contact with the intermediate transfer member; and a control unit which applies an intermediate-transfer-member-bias to the intermediate transfer member and applies a cleaning-member-bias to the cleaning member which is in contact with the intermediate transfer member to thereby control the potential gradient between the cleaning member and the intermediate transfer member, in which when switching the intermediate-transfer-member-bias to be applied to the intermediate transfer member from a first intermediate-transfer-member-bias to a second intermediate-transfer-member-bias and switching the cleaning-member-bias to be applied to the cleaning member from a first cleaning-member-bias to a second cleaning-member-bias, respectively (substantially at the same time), the control unit makes the switching start timing for the intermediate-transfer-member-bias earlier than the switching start timing for the cleaning-member-bias, and effects switching of the cleaning-member-bias stepwise.
Examples of the situation in which switching of the intermediate-transfer-member-bias to be applied to the intermediate transfer member from the first intermediate-transfer-member-bias to the second intermediate-transfer-member-bias and switching of the cleaning-member-bias to be applied to the cleaning member from the first cleaning-member-bias to the second cleaning-member-bias are effected respectively (substantially at the same time) include the time when power is turned on, transition from a standby mode to an image formation mode, transition from the image formation mode to a cleaning mode, transition from the cleaning mode to the image formation mode, transition from the cleaning mode to the standby mode, and transition from a normal-polarity cleaning mode to a reversed-polarity cleaning mode.
Further, the second intermediate-transfer-member-bias and the second cleaning-member-bias are of the same polarity, and when the following inequality: |(second intermediate-transfer-member-bias)| less than |(second cleaning-member-bias)| holds good, in general, discharge is likely to occur between the cleaning member and the intermediate transfer member, so that application of the present invention to such a case is particularly effective.
Further, taking into account the fact that discharge between the cleaning member and the intermediate transfer member is likely to occur when the potential of the cleaning member is higher than the potential of the intermediate transfer member, it is desirable to perform control such that during switching from the first intermediate-transfer-member-bias to the second intermediate-transfer-member-bias, the relationships: xe2x88x92xcex94(lower limit) (0 less than xcex94(lower limit)) less than (cleaning-member-bias)xe2x88x92(intermediate-transfer-member-bias)  less than xcex94(upper limit) (0 less than xcex94(upper limit)), and xcex94(upper limit) less than xcex94(lower limit) are satisfied. Further, it is desirable to perform control such that during switching from the first intermediate-transfer-member-bias to the second intermediate-transfer-member-bias, the relationship: (cleaning-member-bias) less than (intermediate-transfer-member-bias) is satisfied.
Further, generally speaking, when the electrical resistance of the cleaning member is lower than the electrical resistance of the intermediate transfer member which is in contact with the cleaning member, discharge is likely to occur between the cleaning member and the intermediate transfer member, so that application of the present invention to such a case is particularly effective. Specifically, as the cleaning member, it is possible to adopt a metal cleaning roll. In this case, an arrangement may be adopted in which a metal cleaning blade is caused to abut against the cleaning roll.
Further, the present invention is applicable not only to a monochrome image forming apparatus, but also to a multicolor image forming apparatus. That is, it is possible to adopt a construction which has, as the image bearing member, plural image bearing members for different colors and, as the intermediate transfer device, a single intermediate transfer member. Further, it is also possible to adopt a construction which has, as the image bearing member, plural image bearing members for different colors and, as the intermediate transfer device, a first upstream side intermediate transfer member which is in contact with a part of the plural image bearing members, a first downstream side intermediate transfer member which is in contact with a part of the remaining ones of the plural image bearing members, and a second intermediate transfer member which is in contact with the first upstream side intermediate transfer member and the first downstream side intermediate transfer member and to which a toner image is transferred from the first downstream side intermediate transfer member after the transfer of a toner image from the first upstream side intermediate transfer member.
More specifically, the latter type of image forming apparatus may have, as the image bearing member, four image bearing members for yellow, magenta, cyan, and black, and may have, as the transfer device, a first upstream side intermediate transfer member and a first downstream side intermediate transfer member which are respectively in contact with two of the four image bearing members, and a second intermediate transfer member which is in contact with the first upstream side intermediate transfer member and the first downstream side intermediate transfer member and to which a toner image is transferred from the first downstream side intermediate transfer member after the transfer of a toner image from the first upstream side intermediate transfer member.
In accordance with the present invention, it is possible to provide an image forming apparatus capable of preventing generation of an excessive difference in potential between the cleaning member and the intermediate transfer member and of extending the service life of the intermediate transfer member.